To display an image on the face of a CRT, an electron beam is scanned across a phosphorescent coating on the inside of the face in successive horizontal lines which progress from top to bottom. The electron beam intensity is modulated with the information to be displayed so that the phosphorescent coating is caused to glow in a pattern corresponding to the image. In some cases, the image moves, as in television, so that no particular area of the screen is singled out for extended electron beam bombardment. Thus, the electron beam does not land on any given spot for a long enough time to damage the phosphorescent layer. The same is not true, however, when the displayed image is stationary and particularly when it is both stationary and sharply outlined. In such cases, the electron beam is traced repeatedly along the same path across the phosphorescent coated faceplate. The damage that results is what is called "burn-in" and it can leave a permanent and highly undesirable "scar" along wihch the phosphorescent coating is damaged. Consequently, when another image is subsequently sought to be displayed, the outline of the burn-in path may become partially or totally visible.
In one known CRT display anti-burn circuit, a triangular offset voltage waveform is added to each of the normal electron beam deflecting voltages to constantly move the entire electron scan pattern over an infrequently recurring path. The display of stationary images is prevented and the movement of the pattern is sufficiently slow to be imperceptible to the viewer. However, with many commercially available display monitors, e.g., video monitors used in computer terminals, users are not provided access to the deflecting voltages. Furthermore, since the design of the high voltage circuitry used in such monitors varies widely, modifications to monitors provided by different manufacturers would have to be customized. In addition, no means is provided to adapt the characteristics of display movement, e.g., the rate or the extent of the movement, to a particular application.
In view of the foregoing, a recognized problem in the art is the need for a flexible method of operating a display monitor to prevent burn-in without requiring that complex and expensive modifications be made to the monitor itself.